This study develops a three-dimensional finite-element model of a dense-electrode electric capacitance tomography （ECT） sensor under a quasi-static electrostatic field. How electrode-plate axial height, radial angle and interlayer spacing affect the capacitance dynamic range and the uniformity of sensitivity are investigated, and feasible design intervals are final determined. A composite performance objective function is constructed from the image relative error, image correlation coefficient and spatial image error, and the sensor geometry is optimized via response surface methodology (RSM). Results show that the optimized 8×8 dense electrode array markedly improves oil-water two-phase flow imaging: The image correlation coefficient is increased by 9.4%, and the image relative error and spatial image error are reduced by 13.4% and 15.2% respectively. Interaction effects among structural parameters are further analyzed to ensure robustness. The optimized dense array delivers higher resolution and accuracy, demonstrating strong potential of this ECT sensor for multiphase-flow measurement.